1. Field of the Invention
The present invention pertains to optical communications. More particularly, this invention relates to an optical waveguide apparatus with various improvements.
2. Description of the Related Art
As is known, wavelength division multiplexing (WDM) introduces a new way of increasing the capacity of fiber optic communications links. In a WDM system, several independent optical signals, each having a different optical wavelength, are transmitted through a single optical fiber (either single mode or multimode) from the transmitting end of the system. At the receiving end of the WDM system, the different wavelength optical signals are detected and separated in accordance with their wavelengths. Many different techniques have been developed to achieve this wavelength separation. These techniques can be referred to as either bulk optics or integrated optics. In a bulk optical system, light from an optical fiber is first collimated by a lens. The collimated light is then separated into its constituent wavelengths using diffraction gratings, dielectric filters, or prisms. The separated beams are then focused by lenses either onto separate detectors or into separate optical fibers. The optical fibers used in a bulk optical system can be either multimode or single mode optical fibers. Bulk optical systems can be made with low insertion loss and low crosstalk interference.
Disadvantages are, however, associated with the bulk optical systems. One disadvantage of the bulk optical systems is that they are typically large in size. Another disadvantage associated is that the bulk optical systems often consist of expensive elements and typically require labor intensive alignment.
In some prior art integrated optical systems, an optical fiber is directly attached to a dielectric waveguide. In many such systems, a channel waveguide geometry exists which exploits interference and/or diffraction to separate different wavelength constituents into separate waveguides. These systems are only applicable to single mode fibers. In other prior art integrated optical systems, slab waveguide geometries confine the light in one dimension, while allowing the light to diverge in another dimension. Often, an integrated diffraction grating is fabricated in the same substrate as the slab waveguide, to provide the wavelength separation. Such devices are often difficult to fabricate, and typically have a high insertion loss.
One prior solution to solving the above-mentioned problems is shown in FIG. 1. FIG. 1 shows a prior art zigzag patterned dielectric waveguide demultiplexer 10 that separately detects and extracts the optical signals of different wavelengths. In this demultiplexer 10, light containing several constituent wavelengths (e.g., .lambda..sub.1, .lambda..sub.2, .lambda..sub.3, .lambda..sub.4) is coupled directly from an external optical fiber 12 into a dielectric channel waveguide structure 13 of the demultiplexer 10. The structure 13 has a zigzag geometry with dielectric interference filters (e.g., 15a, 15b, 15c, or 15d) or broad-band mirrors (e.g., mirror 14) attached to each vertex of the structure 13.
The zigzag waveguide demultiplexer can be used with either single mode or multimode optical fiber inputs. In addition, it has most of the advantages that the integrated optical wavelength demultiplexers have. This means that, with the exception of the filters and mirrors, a zigzag waveguide demultiplexer is monolithic and can be fabricated by batch process.
One problem of such a prior art zigzag waveguide demultiplexer is that a significant optical loss in the device occurs in the region near the mirror or optical filter vertex where two angled waveguides converge. This effect is shown in FIG. 2. As can be seen from FIG. 2, as light enters the overlap region (i.e., the shaded triangle region 13c) between the input and output waveguides 13a and 13b of the structure 13, the light is no longer confined to the width of a single waveguide. Because the light contains rays of many angles (determined by the critical angle of the core/cladding index difference), divergence occurs in this region 13c. As a result, some fraction of the light is not collected in the output waveguide 13b. FIG. 2 shows two rays (i.e., light rays 18 and 19) which are not collected by the output waveguide 13b. This effect is most pronounced in highly multimode waveguide devices where the Rayleigh range is shorter than the waveguide width.